Chemical detectors have been in use for some time to sense various gases such as hydrogen, oxygen, carbon monoxide, etc. One form of a chemical detector is an electrochemical cell that uses a catalytic electrode so that the gas to be detected is either oxidized or reduced with the exchange of electrons. The flow of current due to the oxidation or reduction of the gas is then detected as a measure of the concentration of the gas to be detected.
A known problem associated with chemical detectors, however, is referred as “drift,” which allows the chemical detectors to lose their sensitivity over time. For example, the working life of an electrochemical cell is determined by the activity of the catalytic electrode that is used to detect chemicals within the detector. This activity is gradually reduced over time by contaminants such that the sensitivity of the detector drifts downward.
Other types of chemical detectors, such as pellistor detectors, biometric detectors, and tin oxide detectors that may be formed as thin film, thick film, sintered or MOSFET devices may have similar problems. If the instrument into which the chemical detector is built is calibrated regularly, adjustment of the chemical detector can compensate for the downward sensitivity drift, and a faulty chemical detector can be replaced immediately.
If the instrument, however, is in a difficult position for servicing, or if calibration of the chemical detector is otherwise not freely available, it is often impossible to confirm that the chemical detector is functioning correctly. Therefore, as the chemical detector reaches the end of its working life, the output of the sensing cell may be low and in chemical alarms may be insufficient to generate an alarm condition. As a result, a situation could arise where toxic levels of chemicals are present, but the chemical detector is incapable of providing the requisite warning.
A substantial effort has been invested in determining a method by which the function of a chemical detector, such as an electrochemical cell, can be checked without the need for an externally generated calibration gas. For example, it has been proposed to use additional electronic components in order to check conductive pathways through the chemical detector. While such methods can uncover broken connections, they do not provide any information on the condition of the electrodes in terms of their ability to react with the chemical to be detected.
External gas sources are often used in industrial settings to calibrate chemical detectors and to correct for drift. Toxic chemical detectors are normally calibrated to measure around the Occupational Exposure Level. For example, for most toxic gases that level, less than 50 ppm, is extremely low. Calibration gas cylinders have a limited shelf life because of the difficulty in preparing a dilute of enough gas/air mixtures, because the materials used to make calibration gas cylinder housings absorb certain toxic gases, and because the mixture can be unstable.
Chemical sensors and biochemical sensors have the same problems as gas sensors and many drift much faster than gas sensors. Electrochemical water content detectors and indirect glucose sensors are examples of the many types of chemical and biochemical sensors. Sensors used for continuous monitoring require dependable periodic testing and calibration. The calibration and testing functions must be easy enough that they are actually used in practice. Self-testing and self-calibration capabilities help make sensor testing and calibration easy.
Oxygen and hydrogen can be generated through the electrolysis of water. The generated oxygen and/or hydrogen can be used for testing and calibrating chemical sensors that rely on the detection of oxygen. The response of the sensor electrodes to hydrogen is similar to that of oxygen, but the electrical current flows in the opposite direction.
The problem with the electrolysis method is that it is difficult to precisely generate a few parts per million (ppm) of gas. Furthermore, some of the gas will dissolve into the water. The amount of gas generated is a function of the voltage, current, temperature, pressure, and other variables. The amount of gas that dissolves into the water is also a function of the voltage, current, temperature, pressure, and other variables. It is difficult to control every variable and therefore it is difficult to generate the gas with the precision required. However, precise generation is necessary for meaningful calibration.
Applications requiring an alarm but not a measurement may not require as accurate a calibration due to loose accuracy requirements and cost considerations, e.g., the UL 2034 regulation for home use CO alarms. Some manufacturers desire a built-in self-testing function for fail-safe purposes. Self-testing and self-calibration are different because self-testing is testing for failures of basic operation, like broken electrodes, while self-calibrating corrects the drift of sensors. Therefore, in self-testing, the amount of gas generated can vary more widely, the heater doesn't have to heat long enough to reach the equilibrium; and the temperature sensor might not be needed. Self-testing can be done more often than self-calibrating because self-testing is easier.
The embodiments discussed herein therefore directly address the shortcomings of the prior art by internally producing a reference gas efficiently, accurately and safely.